The clutches conventionally used have been almost all of the friction type utilizing centrifugal force.
Generally, the aforementioned clutch comprises a clutch boss connected to the engine and a clutch drum arranged concentrically with the clutch boss and rotatable independently of the clutch boss. Interposed between the clutch boss and the clutch drum are a plurality of clutch shoes movable in the radial direction toward the clutch boss and a spring partially pressing the respective clutch shoes inward the radial direction. Thus, when the rotating speed of the clutch boss increases, the clutch shoes will be moved outward in the radial direction against the action of the spring by the centrifugal force applied to them and will be thereby frictionally engaged with the clutch drum to transmit the torque to the clutch drum (for example, see the publication of Japanese Utility Model Application Laid Open No. 81737/1987) .
In the above-mentioned conventional clutch operation is normally smooth at the time of starting the transmission of the torque, but during the time of operation in the low speed rotation range, slippage will be produced. Thus, transmitting efficiency will reduce, and the desired low speed rotation and torque transmission will not be obtained.
Further, the above-mentioned conventional clutch is complicated in its structure, requires such various parts as the clutch boss, clutch drum, clutch shoes, and spring, giving rise to difficulty in its manufacture. In addition, the clutch is expensive and cannot be made small and light.
An object of the present invention is to provide a clutch whereby slippage in the low speed rotation is eliminated, and while it is engaged, the rotation and torque of the engine will be transmitted as they are and without loss.
Another object of the present invention is to provide a clutch wherein the shock at the time of starting the transmission of the torque is small.
Further, a third object of the present invention is to provide a clutch which is simple in structure and can be made small and light and can be provided cheaply.